Rechargeable positive electrodes

ABSTRACT

Disclosed are positive electrodes containing active-sulfur-based composite electrodes. The cells include active-sulfur, an electronic conductor, and an ionic conductor. These materials are provided in a manner allowing at least about 10% of the active-sulfur to be available for electrochemical reaction. Also disclosed are methods for fabricating active-sulfur-based composite electrodes. The method begins with a step of combining the electrode components in a slurry. Next, the slurry is homogenized such that the electrode components are well mixed and free of agglomerates. Thereafter, before the electrode components have settled or separated to any significant degree, the slurry is coated on a substrate to form a thin film. Finally, the coated film is dried to form the electrode in such a manner that the electrode components do not significantly redistribute.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/686,609, filed Jul. 26, 1996 (now U.S. Pat. No. 5,686,201),and entitled RECHARGEABLE POSITIVE ELECTRODES, which is acontinuation-in-part of U.S. patent application Ser. No. 08/479,687 (nowU.S. Pat. No. 5,582,623, issued Dec. 10, 1996, filed Jun. 7, 1995, andentitled METHODS OF FABRICATING RECHARGEABLE POSITIVE ELECTRODES) whichis, in turn, a continuation-in-part of U.S. patent application Ser. No.08/344,384 (now U.S. Pat. No. 5,523,179, issued Jun. 4, 1996, filed Nov.23, 1994, and entitled RECHARGEABLE POSITIVE ELECTRODE). U.S. patentapplication Ser. No. 08/686,609 is incorporated herein by reference forall purposes. In addition, both U.S. Pat. Nos. 5,582,623 and 5,523,179are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

This invention relates generally to positive electrodes characterized byactive-sulfur. The electrodes are preferably rechargeable, and in somepreferred embodiments are constructed in a thin-film format. Variousnegative electrodes, such as, alkali metal, alkaline earth metal,transition metal, and carbon insertion electrodes, among others, can becoupled with the positive electrode to provide battery cells, preferablyhaving high specific energy (Wh/kg) and energy density (Wh/l).

The rapid proliferation of portable electronic devices in theinternational marketplace has led to a corresponding increase in thedemand for advanced secondary batteries. The miniaturization of suchdevices as, for example, cellular phones, laptop computers, etc., hasnaturally fueled the desire for rechargeable batteries having highspecific energies (light weight). At the same time, mounting concernsregarding the environmental impact of throwaway technologies, has causeda discernible shift away from primary batteries and towards rechargeablesystems.

In addition, heightened awareness concerning toxic waste has motivated,in part, efforts to replace toxic cadmium electrodes in nickel/cadmiumbatteries with the more benign hydrogen storage electrodes innickel/metal hydride cells. For the above reasons, there is a strongmarket potential for environmentally benign secondary batterytechnologies.

Secondary batteries are in widespread use in modern society,particularly in applications where large amounts of energy are notrequired. However, it is desirable to use batteries in applicationsrequiring considerable power, and much effort has been expended indeveloping batteries suitable for high specific energy, medium powerapplications, such as, for electric vehicles and load leveling. Ofcourse, such batteries are also suitable for use in lower powerapplications such as cameras or portable recording devices.

At this time, the most common secondary batteries are probably thelead-acid batteries used in automobiles. Those batteries have theadvantage of being capable of operating for many charge cycles withoutsignificant loss of performance. However, such batteries have a lowenergy to weight ratio. Similar limitations are found in most othersystems, such as Ni-Cd and nickel metal hydride systems.

Among the factors leading to the successful development of high specificenergy batteries, is the fundamental need for high cell voltage and lowequivalent weight electrode materials. Electrode materials must alsofulfill the basic electrochemical requirements of sufficient electronicand ionic conductivity, high reversibility of the oxidation/reductionreaction, as well as excellent thermal and chemical stability within thetemperature range for a particular application. Importantly, theelectrode materials must be reasonably inexpensive, widely available,non-toxic, and easy to process.

Thus, a smaller, lighter, cheaper, non-toxic battery is sought for thenext generation of batteries. The low equivalent weight of lithiumrenders it attractive as a battery electrode component for improvingweight ratios. Lithium provides also greater energy per volume than dothe traditional battery standards, nickel and cadmium.

The low equivalent weight and low cost of sulfur and its nontoxicityrenders it also an attractive candidate battery component. Successfullithium/organosulfur battery cells are known. (See, De Jonghe et al.,U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S. Pat. No.5,162,175.)

However, employing a positive electrode based on elemental sulfur in analkali metal-sulfur battery system has been considered problematic.Although theoretically the reduction of sulfur to an alkali metalsulfide confers a large specific energy, sulfur is known to be anexcellent insulator, and problems using it as an electrode have beennoted. Such problems referred to by those in the art include thenecessity of adjoining the sulfur to an inert electronic conductor, verylow percentages of utilization of the bulk material, poor reversibility,and the formation of an insulating sulfur film on the carbon particles(used to impart electronic conductivity to the sulfur electrode) andcurrent collector surface that electronically isolates the rest of theelectrode components. (DeGott, P., "Polymere Carbone-Soufre Synthese etProprietes Electrochimiques," Doctoral Thesis at the Institut NationalPolytechnique de Grenoble (date of defense of thesis: 19 Jun., 1986) atpage 117.)

Similarly, Rauh et al., "A Lithium/Dissolved Sulfur Battery with anOrganic Electrolyte," J. Electrochem. Soc., 126 (4): 523 (April 1979)state at page 523: "Both S₈ and its ultimate discharge product, Li₂ S,are electrical insulators. Thus it is likely that insulation of thepositive electrode material . . . led to the poor results for Li/Scells."

Further, Peramunage and Licht, "A Solid Sulfur Cathode for AqueousBatteries," Science, 261: 1029 (20 Aug. 1993) state at page 1030: "Atlow (room) temperatures, elemental sulfur is a highly insoluble,insulating solid and is not expected to be a useful positive electrodematerial." However, Peramunage and Licht found that interfacing sulfurwith an aqueous sulfur-saturated polysulfide solution converts it froman insulator to an ionic conductor.

The use of sulfur and/or polysulfide electrodes in non-aqueous oraqueous liquid-electrolyte lithium batteries (that is, in liquidformats) is known. For example, Peled and Yamin, U.S. Pat. No.4,410,609, describe the use of a polysulfide positive electrode Li₂S_(x) made by the direct reaction of Li and S in tetrahydrofuran (THF).Poor cycling efficiency typically occurs in such a cell because of theuse of a liquid electrolyte with lithium metal foil, and the Peled andYamin patent describes the system for primary batteries. Rauh et al.,"Rechargeable Lithium-Sulfur Battery (Extended Abstract), J. PowerSources, 26: 269 (1989) also notes the poor cycling efficiency of suchcells and states at page 270 that "most cells failed as a result oflithium depletion."

Other references to lithium-sulfur battery systems in liquid formatsinclude the following: Yamin et al., "Lithium Sulfur Battery," J.Electrochem. Soc., 135(5): 1045 (May 1988); Yamin and Peled,"Electrochemistry of a Nonaqueous Lithium/Sulfur Cell," J. PowerSources, 9: 281 (1983); Peled et al., "Lithium-Sulfur Battery:Evaluation of Dioxolane-Based Electrolytes," J. Electrochem, Soc.,136(6): 1621 (Jun. 1989); Bennett et al., U.S. Pat. No. 4,469,761;Farrington and Roth, U.S. Pat. No. 3,953,231; Nole and Moss, U.S. Pat.No. 3,532,543; Lauck, H., U.S. Pat. Nos. 3,915,743 and 3,907,591;Societe des Accumulateurs Fixes et de Traction, "Lithium-sulfurbattery," Chem. Abstracts, 66: Abstract No. 111055d at page 10360(1967); and Lauck, H. "Electric storage battery with negative lithiumelectrode and positive sulfur electrode," Chem. Abstracts, 80: AbstractNo. 9855 at pages 466-467 (1974).)

High temperature molten sodium-sulfur cells are known and described inU.S. Pat. No. 3,413,150 issued to Kummer et al. on Nov. 26, 1968 andU.S. Pat. No. 3,404,035 issued to Kummer et al. on Oct. 1, 1968 Suchcells employ a solid state separator, typically a ceramic such as analumina. Obviously such cells must be operated at a temperature abovethe melting point of sodium. A lower temperature version of such cellshas been proposed in U.S. Pat. No. 4,268,587 issued to Farrington et al.on May 19, 1981. In that patent, the described cell employed a ceramicseparator as in the high temperature version. It also employed apositive electrode including elemental sulfur, an electronic conductor(e.g., carbon) and a "drop" of liquid ionic conductor.

DeGott, supra, notes at page 118 that alkali metal-sulfur batterysystems have been studied in different formats, and then presents theproblems with each of the studied formats. For example, he notes that an"all liquid" system had been rapidly abandoned for a number of reasonsincluding among others, problems of corrosiveness of liquid lithium andsulfur, of lithium dissolving into the electrolyte provokingself-discharge of the system, and that lithium sulfide forming in thepositive (electrode) reacts with the sulfur to give polysulfides Li₂S_(x) that are soluble in the electrolyte.

In regard to alkali metal-sulfur systems wherein the electrodes aremolten or dissolved, and the electrolyte is solid, which function inexemplary temperature ranges of 130° C. to 180° C. and 300° C. to 350°C., DeGott states at page 118 that such batteries have problems, suchas, progressive diminution of the cell's capacity, appearance ofelectronic conductivity in the electrolyte, and problems of safety andcorrosion. DeGott then lists problems encountered with alkalimetal-sulfur battery systems wherein the electrodes are solid and theelectrolyte is an organic liquid, and by extension wherein the negativeelectrode is solid, the electrolyte is solid, and the positive electrodeis liquid. Such problems include incomplete reduction of sulfur,mediocre reversibility, weak maximum specific power (performance limitedto slow discharge regimes), destruction of the passivating layer of Li₂S as a result of its reaction with dissolved sulfur leading to theformation of soluble polysulfides, and problems with the stability ofthe solvent in the presence of lithium.

DeGott also describes on page 117 a fundamental barrier to goodreversibility as follows. As alkali metal sulfides are ionic conductors,they permit, to the degree that a current collector is adjacent tosulfur, the propagation of a reduction reaction. By contrast, theirreoxidation leads to the formation of an insulating sulfur layer on thepositive electrode that ionically insulates the rest of the composite,resulting in poor reversibility.

DeGott concludes on page 119 that it is clear that whatever format isadopted for an alkali metal-sulfur battery system that the insulatingcharacter of sulfur is a major obstacle that is difficult to overcome.He then describes preliminary electrochemical experiments with acomposite sulfur electrode prepared from a slurry. The slurry wasprepared by mixing the following components in acetonitrile: 46% sulfur;16% acetylene black; and 38% (PEO)₈ /LiClO₄ (polyethylene oxide/lithiumperchlorate). The resulting slurry was then deposited on a stainlesssteel substrate by "capillary action." From those preliminaryexperiments, DeGott concludes on page 128 that it is clear that, evenwhen optimizing the efficiency of the composite electrode (that is, bymultiplying the triple point contacts) that elemental sulfur cannot beconsidered to constitute an electrode for a secondary battery, in an"all solid" format.

Present solid-state lithium secondary battery systems are limited to aspecific energy of about 120 Wh/kg. It would be highly desirable to havea battery system characterized by higher specific energy values.

It would be even more desirable if solid-state batteries havingpractical specific energy values greater than about 150 Wh/kg couldoperate at room temperature. It would be additionally advantageous ifsolid-state batteries having high specific energy and operation at roomtemperature could be reliably fabricated into units with reproducibleperformance values.

In lithium cells wherein a liquid electrolyte is used, leakage of theelectrolyte can leave lithium exposed to the air, where it rapidlyreacts with water vapor and oxygen. Substantial casing can prevent suchreactions and protect users and the environment from exposure tohazardous, corrosive, flammable or toxic solvents but adds unwantedweight to the battery. A solid-state battery would greatly reduce suchproblems of electrolyte leakage and exposure of lithium, and would allowreducing the weight of the battery.

Furthermore, a battery formulation that overcomes the problem of lithiumdepletion described in the prior art, for example, Rauh et al., supra,would have many advantages.

In summary, disadvantages in currently available metal-sulfur batterysystems include poor cycling efficiency, poor reversibility, lithiumdepletion, or operating temperatures above 200° C., among otherproblems. Practitioners in the battery art have long sought asolid-state or gel-state metal-sulfur battery system that would overcomethese limitations.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a battery cell that may becharacterized as including the following elements: (a) a negativeelectrode including a metal or an ion of the metal; (b) a sulfurpositive electrode; and (c) a gel-state or polymeric solid-stateelectrolyte separator electronically separating the positive andnegative electrodes. The positive electrode includes (i) anelectrochemically active material including sulfur in the form of atleast one of elemental sulfur, a sulfide of the metal, and a polysulfideof the metal, and (ii) an electronically conductive material. Inaddition, the positive electrode should exist in a state in which atleast about 10% of the sulfur is accessible to electrons and ioniccharge carriers.

Preferably, the positive electrode also includes a liquid-state ionicconductor. Examples of such liquid-state ionic conductor includesulfolane, dimethyl sulfone, a dialkyl carbonate, tetrahydrofuran,dioxolane, propylene carbonate, ethylene carbonate, dimethyl carbonate,butyrolactone, N-methylpyrrolidinone, tetramethylurea, glymes, ethers, acrown ether, and dimethoxyethane.

The battery cells of this invention may be rechargeable. If so, theyshould preferably meet certain performance criteria. For example, abattery cell of this invention preferably has at least about 10% of thesulfur in the positive electrode accessible to electrons and ioniccharge carriers over at least about 50 successive cycles. Alternatively,at least about 50% of the sulfur should be accessible to electrons andionic charge carriers that over at least about 2 successive cycles.

The electronic conductor of the positive electrode may be carbon or anelectronically conductive polymer, for example. In a particularlypreferred embodiment, the positive electrode includes an polymericdisulfide material in which the disulfide bond is located in thematerial's backbone.

If the electrolyte separator is a gel-state electrolyte separator, itwill include at least about 20% by weight of an aprotic organic liquidimmobilized by the presence of a gelling agent. Examples of such aproticorganic liquids include sulfolane, dimethyl sulfone, a dialkylcarbonate, tetrahydrofuran, dioxolane, propylene carbonate, ethylenecarbonate, dimethyl carbonate, butyrolactone, N-methylpyrrolidinone,tetramethylurea, glymes, ethers, a crown ether, and dimethoxyethane. Ifthe electrolyte separator is a solid-state electrolyte separator, itpreferably a polymeric solid-state electrolyte separator such as apolymer of ethylene oxide.

These and other features of the invention will further described andexemplified in the drawings and detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a flow chart showing the important steps employed inpreparing an electrode in accordance with this invention.

FIG. 2 illustrates a fixed tube apparatus for depositing a film ofslurry on a substrate in accordance with one embodiment of thisinvention.

FIG. 3 illustrates apparatus for continuous slurry film deposition inaccordance with one embodiment of this invention.

FIG. 4 provides a schematic of a Li/electrolyte separator/active-sulfurelectrode cell of this invention.

FIG. 5 illustrates the reversible cycling performance of a lithium cell(Li/amorphous PEO/active-sulfur) of this invention evaluated at 30° C.at an active-sulfur capacity of 330 mAh/gm for each cycle.

FIG. 6 illustrates the availability of the active-sulfur in the positiveelectrode of a lithium cell (Li/amorphous PEO/active-sulfur) of thisinvention evaluated at 30° C.

FIG. 7 illustrates the availability of the active-sulfur in the positiveelectrode of a lithium cell (Li/gel-state electrolyteseparator/active-sulfur) of this invention evaluated at 30° C.

FIG. 8 illustrates the availability of the active-sulfur in the positiveelectrode of a lithium cell (Li/PEO/active-sulfur) of this inventionevaluated at 90° C.

FIG. 9 illustrates the reversible cycling performance of a lithium cell(Li/PEO/active-sulfur) of this invention evaluated at 90° C. at anactive-sulfur capacity of 400 mAh/gm for each cycle.

FIG. 10 illustrates the reversible cycling performance of a lithium cell(Li/PEO/active-sulfur) of this invention evaluated at 90° C.

FIG. 11 illustrates the peak power performance of a lithium cell(Li/PEO/active-sulfur) of this invention evaluated at 90° C.

FIG. 12a is a table illustrating the performance of the cells preparedand operated as described in examples 1-8.

FIG. 12b is a table illustrating the performance of the cells preparedand operated as described in examples 9-15.

FIG. 13 illustrates the peak power performance of a lithium cell(Li/PEO/active-sulfur) of this invention evaluated at 90° C.

FIG. 14a is voltage vs. time cycling profile for a cell (discharged to alevel of 200 mAh/gm active-sulfur and operated at 90° C.) that wasconsistently overcharged by 50% for each cycle.

FIG. 14b is a plot of the end of discharge voltage vs. number ofrecharge cycles for the cell of FIG. 14a.

DETAILED DESCRIPTION

The term "active-sulfur" is defined herein to be elemental sulfur orsulfur that would be elemental if the positive electrode were in itstheoretically fully charged state. Regardless of the initial state ofthe positive electrode, it contains active sulfur if at any time in abattery cycle, it includes elemental sulfur, a sulfide of the negativeelectrode metal, and/or polysulfides of the negative electrode metal.

The term "solid-state" is defined herein to be a material which containsless than 20% by weight of a liquid.

The term "gel-state" is defined herein to be a material containing atleast 20% by weight of a liquid wherein said liquid is immobilized bythe presence of a gelling agent.

The term "component" is defined herein to be (a) positive electrode, (b)electrolyte separator, or (c) negative electrode.

The instant invention provides a positive electrode for solid-state andliquid format battery systems, wherein the positive electrode is basedon active-sulfur which provides high specific energy and power,exceeding that of highly developed systems now known and in use.Solid-state format battery cell means all the components of the batteryare either solid-state or gel-state. It further means that no componentis in a liquid-state. The equivalent weight of the active-sulfur used inthe redox reactions within the battery cells of this invention is 16grams/equivalent (with a lithium metal as the negative electrode,active-sulfur in its theoretically fully discharged state is Li₂ S),leading to a theoretical specific energy of 2800 watthours per kilogram(Wh/kg) for a lithium cell having a average OCV of 2.4 volts. Such anexceedingly high specific energy is very unusual and highly attractive.

Further, the batteries containing the positive electrode of thisinvention can operate at room temperature. The battery systems of thisinvention provide energy to weight ratios far in excess of the presentdemands for load leveling and/or electric vehicle applications, and canbe reliably fabricated into units with reproducible performance values.

This invention can be incorporated in a battery cell which includessolid-state or gel-electrolyte separators. This embodiment excludes theproblem of a battery cell in the liquid format that may sufferelectrolyte leakage. For example, in lithium cells wherein a liquidelectrolyte is used, leakage of the electrolyte can leave lithiumexposed to the air, where it rapidly reacts with water vapor.Substantive casing can prevent such reactions and protects users and theenvironment from exposure to solvents but adds unwanted weight to thebattery. Using a solid-state or gel-state format battery cells greatlyreduces such problems of electrolyte leakage and exposure of lithium,and can cut down on the weight of the battery.

Another embodiment concerns battery cells in a liquid format, which havea solid active-sulfur-based positive electrode of this invention, andwhich have a solid negative electrode that contains carbon (when in thefully discharged state), carbon inserted with lithium or sodium and/or amixture of carbon with lithium or sodium. Such an embodiment canovercome the problem of lithium depletion described in the prior art,for example, Rauh et al., supra.

In accordance with this invention, the active-sulfur-based compositepositive electrode and a battery system constructed with said positiveelectrode are provided. The positive electrodes of this invention arepreferably reversible, and the metal-active-sulfur battery cells arepreferably secondary batteries, and more preferably thin film secondarybatteries.

The invention relates in one aspect to the positive electrode of batterycells wherein both the positive and negative electrodes are solid-stateor gel-state and the electrolyte separator is either a solid-state or agel-state material (see Definition). In another aspect, as indicatedabove, the positive electrode of this invention is used in a batterycell which contains a liquid electrolyte wherein the negative electrodeis solid or gel-state and contains carbon, carbon inserted with lithiumor sodium, or mixtures of carbon with lithium or sodium. However,whatever the format of the battery cells made with the positiveelectrodes of this invention, said positive electrode compriseselemental sulfur as the active component when in the theoretically fullycharged state.

The positive electrode of this invention may be a composite including,in the theoretically fully charged state, elemental sulfur, preferablyan ionically conductive material, and an electronically conductivematerial. Upon discharge, the active-sulfur of the positive electrodereacts with the metal of the negative electrode, and metal sulfides andpolysulfides form. For example, where M is the metal of the negativeelectrode, the overall cell reaction can be described as follows:

    x/zM+S=M.sub.x/z S

wherein M is any metal that can function as an active component in anegative electrode in a battery cell wherein active-sulfur is the activecomponent of the positive electrode; x=0 through x=2; z=the valence ofthe metal; and S is sulfur.

M is preferably selected from the group consisting of alkali metals,alkaline earth metals, and transition metals. M is more preferablyselected from the group consisting of alkali metals, and still morepreferably lithium or sodium. M is most preferably lithium.

More specifically, for example, in a preferred embodiment of thisinvention wherein the negative electrode contains lithium, the overallcell reaction wherein z=1 can be described as follows:

    xLi+S=Li.sub.x S.

When x=2, 100% of the theoretical specific energy of the system has beenreleased.

Upon discharge, the positive electrode becomes a combination of sulfur,metal sulfides and polysulfides, and during the discharging process theproportions of those sulfur-containing components will change accordingto the state of charge. The charge/discharge process in the positiveelectrode is reversible. Similarly, upon recharging, the percentages ofthe sulfur-containing ingredient will vary during the process.

The positive electrode is thus made from an electrode compositioncomprising active-sulfur, an electronically conductive materialintermixed with the active-sulfur in a manner that permits electrons tomove between the active-sulfur and the electronically conductivematerial, and an ionically conductive material intermixed with theactive-sulfur in a manner that permits ions to move between theionically conductive material and the sulfur.

The ionically conductive material of said composite positive electrodeis preferably a polymeric electrolyte, more preferably a polyalkyleneoxide, and further, preferably polyethylene oxide in which anappropriate salt may be added. Additional ionically conductive materialsfor use in the positive electrode include the components described belowin the solid-state and gel-state electrolyte separator.

Exemplary electronically conductive materials of the composite positiveelectrode include carbon black, electronically conductive compounds withconjugated carbon-carbon and/or carbon-nitrogen double bonds, forexample but not limited to, electronically conductive polymers, such as,polyaniline, polythiophene, polyacetylene, polypyrrole, and combinationsof such electronically conductive materials. The electronicallyconductive materials of the positive electrode may also haveelectrocatalytic activity.

The composite sulfur-based positive electrode may further optionallycomprise performance enhancing additives, such as, binders;electrocatalysts, for example, phthalocyanines, metallocenes, brilliantyellow (Reg. No. 3051-11-4 from Aldrich Catalog Handbook of FineChemicals; Aldrich Chemical Company, Inc., 1001 West Saint Paul Avenue,Milwaukee, Wis. 53233 (USA)) among other electrocatalysts; surfactants;dispersants (for example, to improve the homogeneity of the electrode'singredients); and protective layer forming additives (for example, toprotect a lithium negative electrode), such as, organosulfur compounds,phosphates, iodides, iodine, metal sulfides, nitrides, and fluorides,for example LiI, PbS, and HF.

The range of active-sulfur in such electrodes in the theoretically fullycharged state is from 20% to 80% by weight. Said active-sulfur-basedcomposite electrode is preferably processed such that the componentparticles are homogeneously distributed, and segregation and/oragglomeration of the component particles is avoided.

A metal-sulfur battery system constructed with said active-sulfur-basedcomposite positive electrode of this invention should have at least 5%,and more preferably at least 10% availability of the active-sulfur. Thatavailability corresponds to a minimum of 168 mAh per gram of sulfurincluded in the positive electrode. This is based on the theoreticalvalue of 1675 mAh/gm of sulfur at 100% availability.

The electrolyte separator used in combination with the positiveelectrodes of this invention functions as a separator for the electrodesand as a transport medium for the metal ions. Any electronicallyinsulating and ionically conductive material which is electrochemicallystable may be used. For example, it has been shown that polymeric, glassand/or ceramic materials are appropriate as electrolyte separators, aswell as other materials known to those of skill in the art, such as,porous membranes and composites of such materials. Preferably, however,the solid-state electrolyte separator is any suitable ceramic, glass, orpolymer electrolyte such as, polyethers, polyimines, polythioethers,polyphosphazenes, polymer blends, and the like, in which an appropriateelectrolyte salt may be added. In the solid-state, the electrolyteseparator may contain an aprotic organic liquid wherein said liquidconstitutes less than 20% (weight percentage) of the total weight of theelectrolyte separator.

In the gel-state, the electrolyte separator contains at least 20%(weight percentage) of an aprotic organic liquid wherein the liquid isimmobilized by the inclusion of a gelling agent. Any gelling agent, forexample, polyacrylonitrile, PVDF, or PEO, can be used.

The liquid electrolyte for the liquid format batteries using thepositive electrode of this invention, is also preferably an aproticorganic liquid. The liquid format battery cells constructed using thepositive electrodes of this invention would preferably further comprisea separator which acts as an inert physical barrier within the liquidelectrolyte. Exemplary of such separators include glass, plastic,ceramic, polymeric materials, and porous membranes thereof among otherseparators known to those in the art.

Solid-state and gel-state positive electrodes of this invention can beused in solid-state or liquid format batteries, depending on thespecific format of the electrolyte separator and negative electrode.Regardless of the format of the batteries using the positive electrodeof this invention, the negative electrode can comprise any metal, anymixture of metals, carbon or metal/carbon material capable offunctioning as a negative electrode in combination with theactive-sulfur-based composite positive electrode of this invention.Accordingly, negative electrodes comprising any of the alkali oralkaline earth metals or transition metals for example, (the polyetherelectrolytes are known to transport divalent ions such as Zn⁺⁺) incombination with the positive electrode of this invention are within theambit of the invention, and particularly alloys containing lithiumand/or sodium.

Preferred materials for said negative electrodes include Na, Li andmixtures of Na or Li with one or more additional alkali metals and/oralkaline earth metals. The surface of such negative electrodes can bemodified to include a protective layer, such as that produced on thenegative electrode by the action of additives, including organosulfurcompounds, phosphates, iodides, nitrides, and fluorides, and/or an inertphysical barrier conductive to the metal ions from the negativeelectrode, for example, lithium ions transport in lithium phosphate, orsilicate glasses, or a combination of both.

Also preferred materials for said negative electrodes include carbon,carbon inserted with lithium or sodium, and mixtures of carbon withlithium or sodium. Here, the negative electrode is preferably carbon,carbon inserted with lithium or sodium, and/or a mixture of carbon withlithium or sodium. When the negative electrode is carbon, the positiveelectrode is in the fully discharged state, comprising lithium or sodiumsulfides and polysulfides. Particularly preferred negative electrodesfor batteries are lithium inserted within highly disordered carbons,such as, poly p-phenylene based carbon, graphite intercalationcompounds, and Li_(y) C₆ wherein y=0.3 to 2, for example, LiC₆, Li₂ C₆and LiC₁₂. When the negative electrode is carbon, the cells arepreferably assembled with the positive electrode in the fully dischargedstate comprising lithium or sodium sulfides and/or polysulfides. The useof negative electrodes of the carbon, carbon inserted with lithium orsodium, and mixtures of carbon with lithium or sodium with thesolid-state and gel-state positive electrodes of this invention areespecially advantageous when the battery is in the liquid format.

Positive Electrode

The active-sulfur of the novel positive electrodes of this invention ispreferably uniformly dispersed in a composite matrix, for example, theactive-sulfur can be mixed with a polymer electrolyte (ionicallyconductive), preferably a polyalkylene oxide, such as polyethylene oxide(PEO) in which an appropriate salt may be added, and an electronicallyconductive material. Furthermore, the ionically conductive material maybe in a solid-state, a gel-state, or a liquid-state format. In mostcases it will be necessary or desirable to include a suitable polymericelectrolyte, for rapid ion transport within the electrode as is donewith intercalation materials based electrodes. Furthermore, because theactive-sulfur is not electrically conductive, it is important todisperse some amount of an electronically conductive material in thecomposite electrode.

Preferred weight percentages of the major components of theactive-sulfur-based positive electrodes of this invention in atheoretically fully charged state are: from 20% to 80% active-sulfur;from 15% to 75% of the ionically conductive material (which may begel-state or solid-state), such as an aprotic solvent or PEO with salt,and from 5% to 40% of an electronically conductive material, such ascarbon black, electronically conductive polymer, such as polyaniline.More preferably, those percentages are: from 30% to 75% ofactive-sulfur; from 15% to 60% of the ionically conductive material; andfrom 10% to 30% of the electronically conductive material. Even morepreferable percentages are: from 40% to 60% of active-sulfur; from 25%to 45% of the ionically conductive material; and from 15% to 25% of theelectronically conductive material. Another preferred percentage byweight range for the electronically conductive material is from 16% to24%.

Methods of Making a Positive Electrode:

An important feature of this invention is the ability to provideelectrodes having active material (usually active-sulfur and/or apolydisulfide polymer) in intimate contact with both an ionic conductorand an electronic conductor. This facilitates ion and electron transportto and from the active material to allow nearly complete utilization ofthe active material. To this end, the invention provides a method ofproducing electrodes which ensures that at least about 5% of the activematerial in the resulting electrode will be available forelectrochemical reaction. No prior method produces electrodes havingsuch high availability of active-sulfur.

A preferred method of making electrodes in accordance with thisinvention is illustrated in the flow chart of FIG. 1. The method beginswith a step 100 of combining the electrode components (including anelectrochemically active material, an electronic conductor, and an ionicconductor). Next, at a step 102, the mixture is homogenized such thatthe electrode components are well mixed and free of agglomerates.Typically, a slurry will be formed by combining the electrode componentswith a liquid at either step 100 or step 102.

After the electrode components are homogenized and in slurry form, theslurry is coated on a substrate to form a thin film at a step 104. Bestresults will generally be obtained if the slurry is homogenizedimmediately before the film formation at step 104. This ensures that theslurry components have not settled or separated to any significantdegree, thus providing a uniform film with the desired ratio ofelectrode components. Finally, at a step 106, the coated film is driedto form the electrode. The film preferably will be sufficiently thin toallow for rapid drying so that the electrode components do notsignificantly redistribute during drying step 106. The actual filmthickness will, of course, depend upon the amount of liquid used in theslurry.

The components that are combined at step 100 include at least anelectrochemically active insulator (e.g., elemental sulfur or apolydisulfide), an electronically conductive material, and an ionicallyconductive material. Appropriate ratios of these materials are presentedabove for the resulting electrodes. Generally the same ratios may beemployed in the mixture used to make the electrodes. Theelectrochemically active insulator is preferably active-sulfur, but anyelectrochemically active insulator or moderately conductive material maybenefit from the inventive method. The ionic conductor is, as noted,preferably a polymeric ion conductor such as a polyalkylene oxide, andmore preferably PEO or amorphous PEO. However, it may also be a liquidor gel-state material. To increase the conductivity of the ionconductor, it typically will be provided with a salt containing thetransported ion (e.g., a lithium salt such as lithiumtrifluoromethanesulfonimide or lithium perchlorate as described hereinin connection with the electrolyte). The electronic conductor ispreferably a carbon black or an electronically conductive polymer suchas a polyaniline, polythiophene, polyacetylene, polypyrrole, etc. In anespecially preferred embodiment, the electrochemically active materialis active-sulfur, the ionic conductor is PEO (possibly with a smallamount of an appropriate salt), and the electronic conductor is a carbonblack.

In addition to the three above-mentioned "necessary" electrodecomponents, other components that may be added to the mixture include(1) materials to catalyze the transfer of electrons from theelectronically conductive material to the active material, (2) additivesto protect an active metal electrode surface (e.g., lithium or sodiumelectrode surfaces) in cells that employ such electrodes, (3)dispersants, (4) binders, and (5) surfactants.

Materials that catalyze electron transport between the electrochemicallyactive material and the electronic conductor are known in the art andinclude, for example, phthalocyanines, metallocenes, and brilliantyellow. Additives to protect an active metal electrode surface include,for example, organosulfur compounds such aspoly-2,5-dimercapto-1,3,4-thiadiazole, phosphates, iodides, iodine,metal sulfides, nitrides, and fluorides. These materials are believed toprovide a protective layer on the metal electrode surface. By castingthem in the active-sulfur (or other insulator) electrode, small amountsof these protective agents will diffuse across the electrolyte to reactwith the metal electrode and provide the protective layer. Further, adispersant (or dispersants) such as Brij or PEG may also be added to themixture. Such materials reduce a tendency to agglomerate exhibited bysome of the necessary components such as carbon black. Agglomeration, ofcourse, degrades the quality of the resulting electrode by preventingthorough mixing of the components. Other additives are widely used infabricating positive electrodes and are known in the art to have variousbenefits. The use of such additives in formation of electrodes is withinthe scope of this invention.

As noted, the components of the electrode mixture will typically bedispersed in a slurry. Various liquids may be employed in the slurry.Typically, but not necessarily, the liquid will not dissolveactive-sulfur or carbon black. It may, however, dissolve polymericcomponents such as PEO or a polymeric electronic conductor. Preferredliquids evaporate quickly so that the resulting film dries completelyand before redistribution of the components can occur. Examples ofacceptable liquids for the slurry system include water, acetonitrile,methanol, ethanol, tetrahydrofuran, etc. Mixtures of liquid compoundsmay also be employed. In large-scale continuous processes, it may bedesirable to use a relatively low volatility liquid such as water tofacilitate liquid recovery for recycling.

The relative amounts of solid and liquid in the slurry will be governedby the viscosity required for subsequent processing. For example,electrodes formed by a tape casting apparatus may require a differentviscosity slurry than electrodes formed with a Mayer rod. The slurryviscosity will, of course, be governed by such factors as thecomposition and amounts the slurry components, the slurry temperature,and the particle sizes in the slurry. When the mixture includes asoluble ionic conductor such as PEO, the slurry ratio is conventionallydefined in terms of the amount of soluble material to liquid. Amounts ofthe remaining insoluble components are then pegged to the amount ofsoluble material. For PEO-containing electrodes, a preferred range ofconcentrations is between about 30 and 200 milliliters of solvent pergram of PEO.

The exact ordering in which components are added to the slurry is notcritical to the invention. In fact, as illustrated in examples 18 to 20below, various approaches have been found to work with this invention.In one embodiment, for example, the soluble components such as PEO andbrij are first dissolved in the liquid solvent before the insolublecomponents are added. In another exemplary embodiment, all componentsexcept crystalline PEO are dispersed and dissolved before thecrystalline PEO is added. The insoluble components may be added to theslurry sequentially or in a premixed form (i.e., the solid insolublesare mixed before addition to the slurry).

The process of homogenizing the electrode components (step 102 ofFIG. 1) may take a variety of forms in accordance with the presentinvention. The process may vary depending upon whether electrodefabrication is performed batchwise or continuous. For small-scale batchoperations, suitable slurry homogenization apparatus includes stir bars(preferably cross-type stir bars), paint mixers such as rotary blademixers, paint shakers, and shear mixers. Further, any mixing apparatusconventionally used to make "slips" in the ceramic processing arts willbe sufficient for use with this invention. By way of example, some otherbatch mixing systems employ ball milling, tumble mixing, shear mixing,etc. The amount of time required to obtain a suitably homogeneousmixture can be determined by routine experimentation with each of thesepieces of mixing equipment.

Suitably homogeneous mixtures are evidenced by high availability ofactive electrode material in the resulting electrode. It has been foundthat with stir bars, homogenization typically requires about 2 days,whereas with paint mixers and paint shakers homogenization requires lesstime (on the order of a few hours). In scaling up agitators forsuspending solid particles, the torque per unit volume generally shouldbe kept constant. Even so, blending times typically are significantlylonger in larger vessels than in smaller ones and this should befactored into any scale-up.

In large-scale and/or continuous electrode fabrication systems, anindustrial agitator will generally be preferable. Design criteria forsuch systems are well known in the art and are discussed at, forexample, pages 222-264 of McCabe and Smith "Unit Operations of ChemicalEngineering" Third Edition, McGraw Hill Book Company, New York (1976),which reference is incorporated by reference herein for all purposes.Suitable systems include turbine agitators and axial-flow or radial-flowimpellers in tanks or vessels with rounded bottoms. In general, thevessels should not have sharp corners or regions where fluid currentscannot easily penetrate. Further, the system should be designed toprevent circulatory currents which throw solid particles to the outsideof the vessel where they move downward and concentrate. Circulatorycurrents can be mitigated by employing baffles in the system (e.g.,vertical strips perpendicular to the wall of the vessel). Shroudedimpellers and diffuser rings can also be used for this purpose.

Very soon after the slurry is homogenized, it is deposited as a film ona substrate (step 104 of FIG. 1). The exact amount of time betweenhomogenization and deposition will depend upon the physical character ofthe slurry (viscosity, solids concentration, solids particle sizes,etc.). Significant settling and separation of the solids in the slurryis to be avoided. Settling can be slowed by employing (a) smallparticles of low density solids, (b) high concentrations of the solids,and/or (c) highly viscous liquids. Further the particles of the varioussolids components of the slurry can be chosen so that they all settle atabout the same rate, thereby avoiding the problem of segregation. To theextent possible, the slurry, should be delivered to a substrateimmediately after homogenization. For example, slip conditioning andsupply systems such as these provided by EPH Associates, Inc. of Orem,Utah may be used to deliver slurry from a homogenizer directly to asubstrate.

Preferably, the step of slurry film deposition does not rely oncentrifugal, capillary or other forces that tend to exacerbateseparation of the solid slurry components. Thus, for example, proceduresinvolving dipping of a substrate into the slurry generally will not besuitable for use in the present invention.

In accordance with this invention, preferred film formation proceduresinclude (1) deposition onto a substrate via a fixed tube or structuretemporarily defining a chamber above the substrate, (2) spreading via aMayer rod, and (3) spreading via a doctor blade. Deposition via a fixedtube is illustrated in FIG. 2 where a tube 122 is placed against asubstrate 124 with sufficient force to prevent slurry solids fromleaking outside of deposition region 120. The tube 122 preferably ismade from inert materials such as glass tube. It should have a smoothbottom so that it makes good contact and a reasonably impervious sealwith substrate 124. An amount of slurry sufficient to cover region 120is provided through the top of tube 122.

The slurry film also may be applied by spreading. In batch processes, aMayer rod--which is rod of about 1/2 to 1 inch in diameter wound withthin wires--may profitably be used to roll out a thin layer of slurryfilm on the substrate. In continuous or batch processes, a doctor blademay be employed to deliver a thin layer of slurry to a moving sheet ofsubstrate, as explained in more detail below.

Regardless of how the slurry film is applied, it should have a primarydimension, e.g., thickness, that allows for rapid drying. This thicknesswill, of course, depend upon such factors as slurry concentration andliquid volatility. In addition, the slurry film thickness should bechosen so as to produce electrodes of appropriate thickness for theultimate battery application. For example, low power, high energyapplications, such as batteries for pacemakers, may use thickerelectrodes, e.g., up to a few millimeters. In contrast, high powerapplications, such as batteries for power tools or hybrid vehiclesshould employ thinner electrodes, e.g., no more than about 100 μm thick.It should be noted that electrodes of appropriate thickness for lowpower applications may be made by laminating two or more thinnerelectrodes. In this manner, the problem of slow drying associated withthick electrodes can be avoided.

Preferably the substrate on which the slurry is applied is a currentcollector such as a sheet of stainless steel, aluminum, copper,titanium, metallized PET, or other conductive material which will notreact at operating cell conditions. Suitable current collectors may alsotake the form of expanded metals, screens, meshes, foams, etc. as isknown in the art. In alternative embodiments, the substrate may be asheet of inert material that does not adhere to dried electrodematerial. One such suitable substrate material is Teflon®. After theelectrode film is dried, it is peeled away from such substrate and latercontacted to a current collector such as one of the above-mentionedmaterials. Contacting to the current collector may be accomplished byhot pressing, crimping, etc. Alternatively, the current collector can beformed directly on the electrode material by a technique such as metalspraying, sputtering, or other technique known to those of skill in theart.

The process of forming an electrode concludes with a drying step (step106 of FIG. 1). In batch processes, this is preferably accomplished intwo steps: evaporation under ambient conditions for 30 seconds to 12hours, followed by drying under vacuum for about 4 to 24 hours at roomtemperature or an elevated temperature. In continuous processes, dryingmay be accomplished by passing a sheet of electrode/substrate through adrying chamber such as an IR drier. A typical resulting active-sulfurelectrode layer will have a density between about 0.0016 and 0.012 gramsper cm².

It should be understood that if a liquid-state ionic conductor isemployed, such ionic conductor may have to be added to the electrodeafter the drying step. The liquid ionic conductor may be introduced bysimply contacting it with the dried electrode. In some cases, it may behelpful to draw the liquid into the electrode by applying vacuum.

A continuous process for preparing sheets of precipitated polymer willnow be described with reference to FIG. 3. As shown in FIG. 3, a hopper220 dispenses a sheet of homogenized slurry 228 of suitable compositionas described above. The slurry is deposited on a moving substrate sheet222 which passes under blade 226 to produce a thin evenly spread layerof slurry 230 on substrate 222. The lower tip of blade 226 and thesubstrate 222 should be spaced such that slurry layer 230 has athickness facilitating rapid drying as described above.

The substrate sheet 222--which moved along in the continuous process bya roller 232--may be made from a variety of suitable materials includingflexible Teflon or any other release agent. In addition, the substratemay be a material that is intended to be incorporated in the ultimatelyproduced electrode. For example, the substrate may include a metal foil,metallized PET, or screen that is to form a current collector in thefinal electrode. The substrate 222 with slurry layer 230 is directedinto a drying apparatus 248 operated at a temperature sufficient toremove much of the liquid from the slurry. This apparatus may includeone or more dryers such as IR dryers, and it may also have a condenseror other system (not shown) for recovering evaporated slurry liquid.

If the substrate sheet 222 is not a current collector, it may beseparated from the electrode or partially dried electrode after thesubstrate enters drying apparatus 248. The separation can then beaccomplished by providing separate uptake reels for substrate 222(outside drying apparatus 248) and for the resulting electrode sheet. Ofcourse, if the substrate 222 is a current collector or is otherwiseintended to be part of the electrode, no separation is necessary, andthe substrate/electrode laminate is taken up on reel 232 as shown.

In alternative embodiments, the electrode is formed without firstpreparing a slurry. Rather the electrode components--including theelectrochemically-active insulator, the ion conductor, and the electronconductor--are homogenized in a solid state and formed into a sheet asby extrusion or calendaring. The solid state homogeneous mixture mayalso be coated onto a substrate by roll coating, blade coating,extrusion coating, curtain coating, or a related process. In each case,the solid state mixture is caused to flow by application of heat and/orpressure and the resulting viscous or viscoelastic mixture is passedthough a die, a roller, or a blade. In such embodiments, the PEO orother polymeric components should be present in concentrations suitableto allow formation of a viscous or viscoelastic material underconditions encountered in standard polymer processing apparatus. Detailsof suitable polymer processing techniques are found in Middleman,"FUNDAMENTALS OF POLYMER PROCESSING", McGraw-Hill, Inc. 1977 which isincorporated herein by reference in its entirety and for all purposes.In addition to these processing techniques involving flow, alternativetechniques within the scope of this invention include electrostaticdeposition as by processes analogous to xerography. Further, dryprocesses conventionally used in the rubber processing arts may beapplied to form electrodes in accordance with this invention. Becauseeach of the above "dry" techniques do not involve a slurry, no dryingstep is required. Thus, there is less opportunity for the solidelectrode components to segregate or agglomerate after homogenization.

Electrolyte Separators and Liquid Electrolytes

The electrolyte separator for solid-state format battery cellsincorporating the positive electrode of this invention functions as aseparator for the positive and negative electrodes and as a transportmedium for the metal ions. As defined above, the material for such anelectrolyte separator is preferably electronically insulating, ionicallyconductive and electrochemically stable.

When the battery cell is in a solid-state format, all components areeither solid-state or gel-state and no component is in the liquid-state.

The aprotic organic liquids used in the electrolyte separators of thisinvention, as well as in the liquid electrolytes of this invention, arepreferably of relatively low molecular weight, for example, less than50,000 MW. Combinations of aprotic organic liquids may be used for theelectrolyte separators and liquid electrolytes of the battery cellsincorporating the positive electrode of this invention.

Preferred aprotic organic liquids of the battery cells incorporating thepositive electrode of this invention include among other related aproticorganic liquids, sulfolane, dimethyl sulfone, dialkyl carbonates,tetrahydrofuran (THF), dioxolane, propylene carbonate (PC), ethylenecarbonate (EC), dimethyl carbonate (DMC), butyrolactone,N-methylpyrrolidinone, tetramethylurea, glymes, ethers, crown ethers,dimethoxyethane (DME), and combinations of such liquids.

For the battery cells, incorporating the positive electrode of thisinvention, containing a liquid electrolyte wherein the negativeelectrode is carbon-containing, said liquid is also an aprotic organicliquid as described above. Such a format also preferably contains aseparator within the liquid electrolyte as discussed above.

An exemplary solid-state electrolyte separator combined with thisinvention is a ceramic or glass electrolyte separator which containsessentially no liquid. Polymeric electrolytes, porous membranes, orcombinations thereof are exemplary of the type of electrolyte separatorto which an aprotic organic plasticizer liquid could be added accordingto this invention for the formation of a solid-state electrolyteseparator containing less than 20% liquid.

Preferably the solid-state electrolyte separator is a solid ceramic orglass electrolyte and/or solid polymer electrolyte. Said solid-stateceramic electrolyte separator preferably comprises a beta alumina-typematerial, Nasicon or Lisicon glass or ceramic. The solid-stateelectrolyte separator may include sodium beta alumina or any suitablepolymeric electrolyte, such as polyethers, polyimines, polythioethers,polyphosphazenes, polymer blends, and the like and mixtures andcopolymers thereof in which an appropriate electrolyte salt hasoptionally been added. Preferred polyethers are polyalkylene oxides,more preferably, polyethylene oxide.

Exemplary but optional electrolyte salts for the battery cellsincorporating the positive electrode of this invention include, forexample, lithium trifluoromethanesulfonimide (LiN(CF₃ SO₂)₂), lithiumtriflate (LiCF₃ SO₃), lithium perchlorate (LiCIO₄), LiPF₆, LiBF₄,LiAsF₆, as well as, corresponding salts depending on the choice of metalfor the negative electrode, for example, the corresponding sodium salts.As indicated above, the electrolyte salt is optional for the batterycells of this invention, in that upon discharge of the battery, themetal sulfides or polysulfides formed can act as electrolyte salts, forexample, M_(x/z) S wherein x=0 to 2 and z is the valence of the metal.

Negative electrode

For solid-state battery cells incorporating the positive electrode ofthis invention, the negative electrode may.comprise any metal, anymixture of metals, or any carbon or metal/carbon material capable offunctioning as an active component of a negative electrode incombination with said active-sulfur positive electrode. For example, anyof the alkali or alkaline earth metals or transition metals can be used,and particularly mixtures containing lithium and/or sodium.

Preferred materials for said negative electrode for the solid-statebattery cell formats include sodium and/or lithium, and mixtures ofsodium or lithium with one or more additional alkali metals and/oralkaline earth metals. Preferred materials for said negative electrodealso include mixtures of sodium or lithium with one or more elements toform a binary or ternary alloy, such as, Na₄ Pb, lithium-silicon andlithium-aluminum alloys.

A particularly preferred metal for a negative electrode is sodium, or atleast a sodium base alloy (i.e., at least 90% by weight sodium) becauseof its low cost, low equivalent weight and its relatively low meltingpoint of 97.8° C. However, other alkali metals such as Li or K, ormixtures of same with Na may also be used, as desired, to optimize theoverall system.

Also preferred negative electrode materials for the solid-state batterycells incorporating the positive electrode of this invention includecarbon, carbon inserted with lithium or sodium and/or a mixture ofcarbon with sodium or lithium. Exemplary and preferred are Li_(y) C₆(wherein y=0.3 to 2), such as, LiC₆, negative electrodes which comprisegraphite or petroleum coke, for example, graphite intercalationcompounds (GICs), and carbon inserted within highly disordered carbons.The inserted carbon may also be that wherein some carbon has beenalloyed with boron, or wherein the carbon has been prepared from lowtemperature pyrolysis (about 750° C.) of carbon or carbon-siliconcontaining polymers such that the carbon product retains some hydrogenor silicon or both. (See, Sato et al., "A Mechanism of Lithium Storagein Disordered Carbons," Science, 264: 556 (22 Apr., 1994), whichdiscusses very good results with a preferred negative electrode of Liinserted within poly p-phenylene-based (PPP-based) carbon.)

For battery cells using the positive electrode of this invention thatare in liquid formats, the negative electrode is carbon, carbon insertedwith lithium or sodium, or mixtures of carbon and lithium or sodium asdescribed above in relation to solid-state formats, including thepreferable versions of such carbon-containing electrodes. For whateverformat, if the negative electrode contains only carbon, the cell is inthe theoretically fully discharged state, and the positive electrodecomprises lithium or sodium sulfides or polysulfides.

Battery Cells

The battery cells containing the sulfur-based composite positiveelectrodes of this invention can be constructed according toconventional formats as described in the literature. For example, DeJonghe et al., U.S. Pat. No. 4,833,048 and Visco et al., U.S. Pat. No.5,162,175. Such conventional formats are understood to be hereinincorporated by reference.

The novel battery cells incorporating this invention, preferablysecondary cells, more preferably thin film secondary cells, may beconstructed by any of the well-known and conventional methods in theart. The negative electrode may be spaced from the positive sulfurelectrode, and both electrodes may be in material contact with anionically conductive electrolyte separator. Current collectors contactboth the positive and negative electrodes in a conventional manner andpermit an electrical current to be drawn by an external circuit.

Suitable battery constructions may be made according to the known artfor assembling cell components and cells as desired, and any of theknown configurations may be fabricated utilizing the invention. Theexact structures will depend primarily upon the intended use of thebattery unit.

A general scheme for the novel battery cells of this invention in asolid-state format may include a current collector in contact with thenegative electrode and a current collector in contact with the positiveelectrode, and a solid-state electrolyte separator sandwiched betweenthe negative and positive electrodes. In a typical cell, all of thecomponents will be enclosed in an appropriate casing, for example,plastic, with only the current collectors extending beyond the casing.Thereby, reactive elements, such as sodium or lithium in the negativeelectrode, as well as other cell elements are protected.

The current collectors can be sheets of conductive material, such as,aluminum or stainless steel, which remain substantially unchanged duringdischarge and charge of the cell, and which provide current connectionsto the positive and negative electrodes of the cell. The positiveelectrode film may be attached to the current collector by directlycasting onto the current collector or by pressing the electrode filmonto the current collector. Positive electrode mixtures cast directlyonto current collectors preferably have good adhesion. Positiveelectrode films can also be cast or pressed onto expanded metal sheets.Alternately, metal leads can be attached to the positive electrode filmby crimp-sealing, metal spraying, sputtering or other techniques knownto those skilled in the art. The sulfur-based positive electrode can bepressed together with the electrolyte separator sandwiched between theelectrodes. In order to provide good electrical conductivity between thepositive electrode and a metal container, an electronically conductivematrix of, for example, carbon or aluminum powders or fibers or metalmesh may be used.

A particularly preferred battery cell comprises a solid lithium orsodium electrode, a polymeric electrolyte separator, either solid-stateor gel, preferably a polyalkylene oxide, such as, polyethylene oxide,and a thin-film composite positive electrode containing an elementalsulfur electrode (that is in the theoretically fully charged state), andcarbon black, dispersed in a polymeric electrolyte. Optionally theelectrolyte separator in such a preferred battery call can comprise anelectrolyte salt.

Sulfur Electrodes having a Liquid Ionic Conductor

As mentioned, the battery cells of this invention may include a gelstate or solid state electrolyte separator. In one preferred embodiment,the solid state separator is made from a polymeric material. Theseseparators should be distinguished from ceramic electrolyte separators(which are also solid state separators) such as those employed in hightemperature sodium-sulfur cells and related battery systems. The cellsof this invention employing gel and polymeric solid state electrolyteseparators have at least the following advantages: (1) an electrolytethat is flexible and easy to laminate with many different positive andnegative electrodes, (2) an electrolyte that is compatible with manyliquid and gel state ionic conductors used with positive electrodes ofthis invention, and (3) an electrolyte that is chemically well suitedfor transport of lithium ions and other cell discharge species.

In some embodiments of this invention, the positive electrode used with.a gel or polymeric solid state electrolyte separator includes at least aliquid ionic conductor, an electronic conductor, and active sulfur(e.g., elemental sulfur, a sulfide of the negative electrode metal,and/or one or more polysulfides of the negative electrode metal). Theelectronic conductor may be carbon or other material discussed above.Further, the positive electrode may be provided with one or moreadditives of the types discussed above.

Preferably, the ionic conductor is one or combinations of the followingaprotic organic liquids or related compounds: sulfolane, dimethylsulfone, dialkyl carbonates, tetrahydrofuran (THF), dioxolane, propylenecarbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC),butyrolactone, N-methylpyrrolidinone, tetramethylurea, glymes, ethers,crown ethers, and dimethoxyethane (DME). The liquid ionic conductor inthe positive electrode may optionally include a salt such as one or moreof the following: lithium trifluoromethanesulfonimide (LiN(CF₃ SO₂)₂),lithium triflate (LiCF₃ SO₃), lithium perchlorate (LiClO₄), LiPF₆,LiBF₄, LiAsF₆, as well as corresponding salts depending on the choice ofmetal for the negative electrode (e.g., sodium salts). It should beunderstood that such electrolyte salts are optional because the sulfurdischarge products of this invention naturally increase the ionicconductivity of the liquid. Upon discharge of the battery, the metalsulfides or polysulfides formed can act as electrolyte salts.

In general, the positive electrode may include between from about 20% to80% active-sulfur; from about 15% to 75% of the ionically conductivematerial, and from about 5% to 40% of an electronically conductivematerial, such as carbon black, electronically conductive polymer, suchas polyaniline. More preferably, those percentages are: from about 30%to 75% of active-sulfur; from about 15% to 60% of the ionicallyconductive material; and from about 10% to 30% of the electronicallyconductive material. Even more preferable percentages are: from about40% to 60% of active-sulfur; from 25% to 45% of the ionically conductivematerial; and from about 15% to 25% of the electronically conductivematerial. Another preferred percentage by weight range for theelectronically conductive material is from about 16% to 24%.

In a cell including both a liquid ionic conductor (in the electrode) anda gel or polymeric solid state electrolyte separator as described, thepositive electrode components may reside in a separate cell compartmentdefined on at least one side by the electrolyte separator. and onanother side by a current collector such as a foil. Alternatively, theelectrode may be defined by a carbon felt, a carbon paper, or othermatrix of electronically conductive material. In such cases, the activesulfur may be interspersed throughout the matrix to facilitate chargetransfer to and from the active sulfur.

It is important that the positive electrode be designed to make theactive sulfur available to electronic and ionic charge carriers. Thisallows high utilization of the active sulfur in the cell. To this end,the electronic conductor in the positive electrode should form aninterconnected matrix so that there is always a clear current path fromthe positive current collector to any position in the electronicconductor. The interconnected matrix of the electronic conductor shouldalso be sufficiently "open" that there is room for precipitatedelectroactive species to deposit on the matrix. Suitable electronicconductors may be fibrous materials such as a felt or paper. Examples ofsuitable materials include a carbon paper from Lydall Technical PapersCorporation of Rochester, N.H. and a graphite felt available fromElectrosynthesis Company of Lancaster, N.Y.

Any binder employed in the positive electrode should not prevent contactbetween the electronic conductor and the electroactive species. Forexample, the binder should not provide so much wetting that precipitatedsulfur particles and/or the current collector are completely wetted andtherefore unable to exchange electrons.

Specific examples of suitable polymeric solid state electrolyteseparators include polyethers, polyimines, polythioethers,polyphosphazenes, and the like, as well as copolymers thereof andpolymer blends. Preferably, an appropriate electrolyte salt is added toenhance conductivity. In a particularly preferred embodiment, thepolymer separator is a polyalkylene oxide such as polyethyleneoxide.Specific examples of suitable gel state electrolyte separators includegelled aprotic solvents such as gelled versions of the aprotic solventslisted as appropriate ionic conductors for the positive electrode. Forexample, a gel state electrolyte separator of this invention may includea gelling agent in combination with at least one of sulfolane, dimethylsulfone, a dialkyl carbonate, tetrahydrofuran, dioxolane, propylenecarbonate, ethylene carbonate, dimethyl carbonate, butyrolactone,N-methylpyrrolidinone, tetramethylurea, glymes, ethers, a crown ether,and dimethoxyethane.

Preferably, liquid ionic conductor cells as described are secondarycells having high sulfur utilizations. Unlike primary cells whichdischarge only once, the secondary cells of this invention cycle betweendischarge and charge at least two times. Preferably, secondary cells ofthis invention will cycle at least 50 times, with each cycle having asulfur utilization (measured as a fraction of 1675 mAh/g sulfur outputduring the discharge phase of the cycle) of at least about 10%. Morepreferably, at least 50 cycles will have a minimum sulfur utilization ofat least about 20% (most preferably at least about 30%). Alternatively,the secondary cells of this invention will cycle at least two times,with each cycle attaining at least 50% utilization of sulfur in thepositive electrode.

As used herein, "utilization" assumes that if all elemental sulfur in anelectrode is fully utilized, the electrode will produce 1675 mAh/g ofsulfur. That is, 100% utilization corresponds to 1675 mAh/g of thesulfur in the cell, and 10% utilization corresponds to 167.5 mAh/g ofsulfur in the cell.

Unlike traditional liquid electrolyte separator cells, the gel or solidstate separator cells of this invention are configured to resistself-discharge. Such self-discharge results when chemical species in thepositive electrode dissolve in the electrolyte and migrate to thenegative electrode where they chemically react to become unusable forgeneration of electrical energy. Such self-discharge mechanisms can becountered in various ways. For example, the negative electrode can beprovided with a protective film which blocks dissolved positiveelectrode species from reacting with the negative electrode. Thisprotective layer should be conductive to lithium ions and help preventthe formation of lithium dendrites or "mossy" lithium on repeatedcycling. It can be produced by the action of additives, includingorganosulfur compounds, phosphates, iodides, nitrides, and fluorides.The protective layer may also be preformed from an inert physicalbarrier conductive to the metal ions from the negative electrode.Examples of such preformed protective layers include lithium phosphate,lithium silicate glasses, polymers, or a combinations of these. In aparticularly preferred embodiment, the protective layer ispoly(1-trimethylsilyl-1-propyne) ("PTMSP").

Overcharge Protection

The Li/S system exhibits significant tolerance to overcharge, which maybe attributed to an intrinsic overcharge mechanism. While not wishing tobe bound by theory, it is believed that oxidized overcharge productsproduced in the positive electrode near or at the end of the chargingcycle travel to the negative electrode and affect the surface of thelithium. These overcharge products are reactive with an negativeelectrode passivation layer which consists, in part or wholly, of lessoxidized lithium sulfides. A reaction between the more oxidizedovercharge products and the passivation layer compounds reduces theovercharge products. Reduced species may then travel to the positiveelectrode where they may be reoxidized before returning to the negativeelectrode. Thus, it appears that a redox shuttle mechanism thatgradually removes or reduces the thickness of the negative electrodepassivating layer protects the battery from overcharge.

Regarding the species involved in the protective redox shuttle, it isbelieved that the more oxidized overcharge products from the positiveelectrode include lithium sulfides such as Li₂ S_(x), in which x>3 butprobably 6<x<20. The reactivity of the sulfides depends on the value ofx in Li₂ S_(x), with the reactivity increasing significantly when x>6.During overcharge, the reactive oxidized positive electrode products(e.g., those products having a value of x that is greater than 6) whichare soluble in the electrolyte travel to the negative electrode wherethey are rapidly reduced by less oxidized species such as Li₂ S from thepassivation layer. The more stable reduced products (e.g., thoseproducts having a value of x that is less than 6), then travel back tothe positive electrode where they may be oxidized to the more reactivespecies in a continuation of the described redox cycle. This sulfideredox shuttle mechanism is therefore intrinsic to Li/S cells in whichLi₂ S_(x) is soluble to some extent in the electrolyte.

Because the less oxidized species are more stable, the cells remainstably charged with, at least, x>6 reducing the self discharge rate to alow level.

The above described mechanism should be contrasted with conventionalovercharge protection schemes in which a parasitic additive is providedto the cell. Such additives are chosen based upon characteristicvoltages at which they are oxidized and reduced. The protectionmechanisms of the present invention are, in contrast, based upon acomposition rather than voltage. Only after all sulfur species aresufficiently oxidized (e.g., with x being approximately 5 or greater),will the overcharge mechanism of this invention be activated.

The invention's improvement over conventional overcharge protectionmechanisms can be understood by considering how such mechanisms operate.By using a conventional additive in a cell, the cell voltage ismaintained below a characteristic voltage. Thus, if the cell isovercharged or very rapidly charged, the cell voltage is maintained at apresumably safe level by the reaction of the additive.

As an example, consider the ferrocene compounds that are widely used asovercharge protecting additives. A given ferrocene may be oxidized at avoltage of about 3 volts (versus the lithium negative electrode). Whenduring charge all positive electrode material has been fully charged,the cell voltage may approach 3 volts. If the ferrocene is present, itwill react at this point. Specifically, it will be oxidized at thepositive electrode, travel to the negative, be reduced there, andshuttle back to the positive electrode. This shuttle redox mechanismwill protect the cell from attaining too high a voltage.

However, sometimes during rapid charging the cell voltage may slightlyexceed the point at which the additive reacts, even though the cell hasnot been charged to full capacity. Then the charging current will beshunted to the ferrocene redox reaction and beneficial charging of thecell will cease until the cell voltage is lowered to the level where theferrocene no longer reacts. This problem will not arise in thecomposition dependent mechanism of the present invention. As long asthere is additional active-sulfur in a limited oxidation state, thecharging reaction will proceed--regardless of cell voltages slightlyexceeding the normal fully charged cell voltage.

It should be understood that the above-described overcharge protectionsystem should work with many different cells employing a sulfurelectrode (not just the above-described Li/Sulfur cells). In fact, mostalkali metal-sulfur cells, alkaline earth metal-sulfur cells, and othermetal-sulfur cells which have produced passivation layers and overchargeproducts that can be expected to interact via a redox shuttle similar tothat described above specifically for the lithium-sulfur system. Suchcells might include sodium-sulfur cells, potassium-sulfur cells,magnesium-sulfur cells, aluminum-sulfur cells, calcium-sulfur cells,etc. Of course, alloys of the alkali and alkaline earth metals may alsobe employed as the negative electrodes in these alkali metal-sulfurcells. Such alloys include those described above (e.g.,lithium-magnesium alloys, lithium-aluminum alloys, lithium-lead alloys,sodium-aluminum alloys, sodium-lead alloys, sodium-silicon alloys, andthe like).

To the extent that the sulfur electrodes of this invention are used withalkali metal intercalation negative electrodes (i.e., carbon-alkalimetal electrodes), the systems may benefit from selective separators inwhich sulfides are insoluble. In one example, such separators would beconductive only to Li⁺ single ions. Alternatively, the separator mayinclude two materials: a layer of less selective material as used in aconventional lithium metal cell together with a thin film of materialconductive only to Li⁺ single ions.

Preferably, the overcharge protection of this invention limits the cellvoltage during overcharge to a safe level that does not substantiallyexceed the normal fully charged cell voltage. Thus, the overchargevoltage should remain below the level at which damage is done to cellcomponents. For example, the overcharge voltage should not cause (i) theelectrolyte to electrolyze, (ii) the current collectors to rapidlycorrode, (iii) the cell separator to rapidly degrade, and (iv) thepositive electrode to be irreversibly damaged. Preferably, theovercharge cell voltage will not exceed the normal fully charged cellvoltage by more than about 4 volts, more preferably by not more thanabout 2 volts, and most preferably by not more than about 1 volt. In onespecific embodiment, a lithium-sulfur cell having a fully charged normalcell voltage of at least about 2.2 volts had an overcharge cell voltageof not more than about 2.3 to 2.4 volts (see FIG. 14a described below).

It should be understood that the value of the "fully charged cellvoltage" is not necessarily constant between any two metal-sulfur cellsor is even constant for a given cell over that cell's life. Obviously,there will be some chemical and/or structural variations from cell tocell that will cause the fully charged cell voltage to vary. Inaddition, metal-sulfur cells sometimes exhibit gradual (or abrupt)changes in cell voltage over normal cycling. In all cases, theovercharge protection afforded by the present invention can becharacterized as a limitation in the deviation from the value of thefully charged cell voltage.

Operating Temperatures

The operating temperature of the battery cells incorporating the novelpositive electrode of this invention is preferably 180° C. or below.Preferred operating temperature ranges depend upon the application.Exemplary preferred operating temperature ranges include from -40° C. to145° C.; from -20° C. to 145° C.; from -20° C. to 120° C.; and from 0°C. to 90° C. Most preferably for many applications, the cellsincorporating this invention operate at ambient or above-ambienttemperatures.

Different embodiments of this invention can provide different preferredoperating temperature ranges. The choice of electrolyte can influencethe preferred operating temperature ranges for the batteriesincorporating the positive electrode of this invention. For example,when conventional PEO is used the preferred operating range is 60° C. to120° C.; whereas when amorphous PEO (aPEO) is used, the battery can berun at room temperature, or in a range of 0° C. to 60° C.

Gel formats also provide for lower operating temperature ranges.Exemplary battery cells using the positive electrode of this inventioncontaining, for example, polymeric electrolyte separators withincreasingly greater percentage of a aprotic organic liquid immobilizedby the presence of a gelling agent, can provide for increasingly loweroperating temperature ranges. An exemplary operating temperature rangefor a solid-state battery having gel-state components of this inventionwould be from about -20° C. to about 60° C.

A battery with a liquid separator and an negative electrode comprisingcarbon, inserted carbon and/or a mixture of carbon and lithium or sodiumcan operate at a preferred temperature range of from -40° C. to 60° C.

The high temperature operating range of the battery cells based on thepositive electrode of this invention can be limited by the melting pointof either a solid electrode or a solid electrolyte. Thus sodium negativeelectrodes are limited to temperatures below 97.8° C., but sodium alloyelectrodes, such as Na₄ Pb, can be used in a solid form at well over100° C.

The Li/S cell of this invention differs in several important aspectsfrom the many high temperature sodium-sulfur cells such as those basedon the Na/sulfur system first described by Kummer and Weber of the FordMotor Company (U.S. Pat. Nos. 3,404,035 and 3,413,150). Most notably,the positive electrodes of such Na/S systems are not provided with aseparate ion conductor such as the ion conductors described above.Further, in such Na/S systems, the positive electrodes consist of moltenalkali polysulfides, requiring these cells to operate at temperaturesabove 250° C. The positive electrodes of such cells contain only moltensulfides and carbon. In addition, the high temperature of operation alsomakes the sodium negative electrode molten, thus requiring a ceramicelectrolyte separator. Operation of such cells below 100° C. is notpossible.

Specific Energy and Specific Power

The practical specific energies of the secondary cells utilizing thisinvention are preferably greater than 65 watt-hours per kilogram(Wh/kg), more preferably greater than 100 Wh/kg, still more preferablygreater than 150 Wh/kg, even more preferably greater than 200 Wh/kg, andstill even more preferably greater than 250 Wh/kg. While cells havingspecific energies in the above ranges are preferred for manyapplications, these ranges should not be viewed as limiting theinvention. In fact, the cells of this invention can be expected toachieve specific energies in excess of 850 Wh/kg. Thus, for someapplications, a preferred practical specific energy range of thebatteries incorporating this invention is from about 100 Wh/kg to about800 Wh/kg.

The practical steady-state specific power of the secondary cellsutilizing this invention are preferably greater than 20 watts perkilogram (W/kg), more preferably greater than 50 W/kg, still morepreferably greater than 100 W/kg, even more preferably greater than 150W/kg, and still even more preferably greater than 250 W/kg. It isenvisioned that with battery construction optimized for power, thesteady-state power of this invention can exceed 750 W/kg. A preferredpractical specific energy range of the batteries incorporating thisinvention is from about 50 W/kg to about 500 W/kg. The peak and pulsepower performances would be many times greater than the steady-statepower.

Cells made with lithium negative electrodes, solid-state or gel-stateelectrolyte separators, and positive electrodes made with elementalsulfur, polyethylene oxide (or modified polyethylene oxide) and carbonparticles were constructed to test the performance of the batteries ofthis invention. Examples of these tests will serve to further illustratethe invention but are not meant to limit the scope of the invention inany way.

EXAMPLE 1 Solid-state cell: Cycling performance at an active-sulfurcapacity of 330 mAh/gm for each recharge cycle evaluated at 30° C.

A positive electrode film was made by mixing 45% (percentage by weight)elemental sulfur, 16% carbon black, amorphous polyethylene oxide (aPEO)and lithium trifluoromethanesulfonimide (wherein the concentration ofthe electrolyte salt to PEO monomer units (CH₂ CH₂ O) per molecule ofsalt was 49:1), and 5% 2,5-dimercapto-1,3,4-dithiadiazole in a solutionof acetonitrile (the solvent to PEO ratio being 60:1 by weight). Thecomponents were stir-mixed for approximately two days until the slurrywas well mixed and uniform. A thin positive electrode film was castdirectly onto stainless steel current collectors, and the solvent wasallowed to evaporate at ambient temperatures. The resulting positiveelectrode film weighed approximately 0.0028 gm per cm².

The polymeric electrolyte separator was made by mixing aPEO with lithiumtrifluoromethanesulfonimide, (the concentration of the electrolyte saltto PEO monomer units (CH₂ CH₂ O) per molecule of salt being 39:1) in asolution of acetonitrile (the solvent to polyethylene oxide ratio being15:1 by weight). The components were stir-mixed for two hours until thesolution was uniform. Measured amounts of the separator slurry were castinto a retainer onto a release film, and the solvent was allowed toevaporate at ambient temperatures. The resulting electrolyte separatorfilm weighed approximately 0.0144 gm per cm².

The positive electrode film and polymeric electrolyte separator wereassembled under ambient conditions, and then vacuum dried overnight toremove moisture prior to being transferred into the argon glove box forfinal cell assembly with a 3 mil (75 micron) thick lithium negativeelectrode film (FMC/Lithco, 449 North Cox Road, Box 3925 Gastonia, N.C.28054 (USA)).

A schematic of the cell layers are shown in FIG. 4. Once assembled, thecell was compressed at 2 psi and heated at 40° C. for approximately 6hours. After heating the layers of lithium, electrolyte separator, andthe positive electrode were well adhered.

The cell was then evaluated with a battery tester (Maccor Inc., 2805West 40th Street, Tulsa, Okla. 74107 (USA)) inside the glove box at 30°C. That procedure was performed to eliminate any contamination problemsof the lithium.

The cell was cycled to a constant capacity corresponding to delivering330 mAh per gram of the active-sulfur in the positive electrode film.The rates used were 100-20 μA/cm² for discharging and 50-10 μA/cm² forcharging to cutoff voltages of 1.8 and 3.0 volts, respectively.

FIG. 5 shows the end of the discharge voltage of the cell after eachrecharge cycle. As evident from the graph, the cell performance is veryconsistent.

EXAMPLE 2 Solid-state cell: Total discharge capacity to 900 mAh/gm ofactive-sulfur evaluated at 30° C.

A cell identical to the one described in Example 1 was discharged to 1.8volts at current densities of 100-20 μA/cm² at 30° C. to determine thetotal availability of the active-sulfur in the film. The resultingdischarge curve is seen in FIG. 6. The total capacity delivered by thisfilm was in excess of 900 mAh per gram of the active-sulfur, that is, autilization of 54% of the available active-sulfur, wherein 100% would be1675 mAh/gm.

EXAMPLE 3 Solid-state cell having gel-state components: Total dischargecapacity to 900 mAh/gm of active-sulfur evaluated at 30° C.

A positive electrode film similar to the one described in Example 1 wasmade with a composition of 50% (percentage by weight) elemental sulfur,16% carbon black, amorphous polyethylene oxide (aPEO) and lithiumtrifluoromethanesulfonimide (at a 49:1 concentration).

The electrolyte separator used was a gel made inside the glove box toavoid moisture and oxygen contamination. A starting solution consistingof 10% (weight percentage) of lithium trifluoromethanesulfonimide and90% of tetraethylene glycol dimethylether (tetraglyme) was made. Then asolvent of 90% tetrahydrofuran (THF) was mixed with 10% of the startingsolution. 5.6% Kynar Flex 2801 (Elf Atochem of North America, Inc.,Fluoropolymers Department, 2000 Market Street, Philadelphia, Pa. 19103(USA)), a gelling agent (PVDF), was added to the mixture.

The mixture was stirred for a few minutes and then left standing for 24hours so that the components were absorbed into the Kynar,. The mixturewas stirred again for a few minutes to homogenize the components andthen heated for 1 hour at 60° C. Electrolyte separator films were castonto a release film, and the THF solvent was allowed to evaporate atambient temperatures. The resulting electrolyte separator film weighedapproximately 0.0160 gm per cm².

The resulting cell comprising the positive electrode film, the gel-stateelectrolyte separator film, and the lithium negative electrode wastested at the same conditions as the cell described in Example 2. Thetotal capacity delivered by this film was also in excess of 900 mAh pergram of the active-sulfur, that is, a utilization of 54% of theavailable active-sulfur, wherein 100% would be 1675 mAh/gm as shown inFIG. 7.

EXAMPLE 4 Solid-state cell: Total discharge capacity to 1500 mAh/gm ofsulfur evaluated at 90° C.

A positive electrode film similar to the one described in Example 1 wasmade for use at above ambient temperatures with a composition of 50%(weight percentage) elemental sulfur, 16% carbon black, polyethyleneoxide (900,000 molecular weight) and lithiumtrifluoromethane-sulfonirmide (a 49:1 concentration).

The solid-state electrolyte separator used was cast from a slurry of900,000 MW PEO in acetonitrile without any additional electrolyte salts.The resulting electrolyte separator film weighed approximately 0.0048 gmper cm².

The cell was assembled as described in Example 1. Once assembled, thecell was compressed at 2 psi and heated at 90° C. for approximately 6hours. The cell was tested at 90° C. inside a convection oven located inthe glove box. The cell was discharged to 1.8 V at rates of 500 to 100μA/cm².

The capacity relative to the active-sulfur versus the voltage duringdischarge is shown in FIG. 8. The total capacity delivered by this filmwas also in excess of 1500 mAh per gram of the active-sulfur, that is, autilization of 90% of the available active-sulfur, wherein 100% would be1675 mAh/gm.

EXAMPLE 5 Solid-state cell: Cycling performance at a sulfur capacity of400 mAh/gm for each cycle evaluated at 90° C.

A positive electrode film similar to the one described in Example 4 wasmade with a composition of 50% (weight percentage) elemental sulfur, 24%carbon black, polyethylene oxide (900,000 molecular weight) and lithiumtrifluoromethanesulfonimide (a 49:1 concentration). The electrolyteseparator is also the same as described in Example 4. The cell wastested at 90° C. and cycled to a constant capacity corresponding todelivering 400 mAh/gm of the active-sulfur in the positive electrodefilm. The rate used was 500 μA/cm² for discharge to 1000-500 μA/cm² forcharging at cutoff voltages of 1.8 and 2.6 volts, respectively.

FIG. 9 shows the end of the discharge voltage of the cell after eachrecharge cycle. As evident from the graph, the cell performance is veryconsistent.

EXAMPLE 6 Solid-state cell: Cycling performance for each cycle to acutoff voltage of 1.8 V evaluated at 90° C.

A positive electrode film identical to the one described in Example 4was made. The electrolyte separator is also the same as described inExample 4. The cell was tested at 90° C. and cycled between voltagelimits between 1.8-2.6 volts. The rates used were 500-100 μA/cm² forcharging. FIG. 10 shows the delivered capacity after each recharge. Asevident from this graph most recharge cycles delivered above 1000 mAhper gram of the active-sulfur used in the positive electrode film.

EXAMPLE 7 Solid-state cell: Peak power performance evaluated at 90° C.

A positive electrode film similar to the one described in Example 4 wasmade with a composition of 50% (weight percentage) elemental sulfur, 16%carbon black, polyethylene oxide (900,000 molecular weight) and lithiumtrifluoromethanesulfonimide (a 49:1 concentration). The electrolyteseparator is also the same as described in Example 4. The cell wastested at 90° C. and pulse discharged for a 30 second duration or to acutoff voltage of 1.2 V. The discharge rates ranged from 0.1-3.5 mA/cm².The pulse power (W/kg) delivered by the positive electrode film versusthe current density is shown in FIG. 11. As seen from the plot, anextraordinarily high pulse power of 3000 W/kg is capable of beingattained.

EXAMPLE 8

A cell was tested under the conditions described in Example 5 above,except that the cell was cycled to a constant capacity corresponding todelivering 200 mAh/gm of the active-sulfur in the positive electrodefilm. The electrode was prepared from 50% elemental sulfur, 16% carbon,and the balance 900,000 MW PEO. A film of electrode material was formedwith a Mayer rod onto a current collector. The separator was like theone in example 4 with 900,000 MW PEO and formed with a Mayer rod.

EXAMPLE 9

A cell was tested under the conditions described in Example 5 above,except that the cell was cycled to a constant capacity corresponding todelivering 300 mAh/gm of the active-sulfur in the positive electrodefilm. The electrode was prepared from 45% elemental sulfur, 16% carbon,5% 2,5-dimercapto-1,3,4-dithiadiazole, and the balance 900,000 MW PEO. Afilm of electrode material was formed with a Mayer rod onto a currentcollector. The separator was like the one in example 4 with 900,000 MWPEO and formed with a Mayer rod.

EXAMPLE 10

A cell was tested under the conditions described in Example 5 above,except that the cell was cycled to a constant capacity corresponding todelivering 400 mAh/gm of the active-sulfur in the positive electrodefilm. The electrode was prepared from 45% elemental sulfur, 16% carbon,5% 2,5-dimercapto-1,3,4-dithiadiazole, and the balance 900,000 MW PEO. Afilm of electrode material was formed with a Mayer rod onto a currentcollector. The separator was like the one in example 4 with 900,000 MWPEO and formed with a Mayer rod.

EXAMPLE 11

A cell was tested under the conditions described in Example 5 above,except that the cell was cycled to a constant capacity corresponding todelivering 600 mAh/gm of the active-sulfur in the positive electrodefilm. The electrode was prepared from 50% elemental sulfur, 24% carbon,1% PbS, and the balance 900,000 MW PEO. A film of electrode material wasdirectly cast onto a current collector. The separator was like the onein example 4 with 900,000 MW PEO and formed with a Mayer rod.

EXAMPLE 12

A cell was tested under the conditions described in Example 6 above. Theelectrode was prepared from 50% elemental sulfur, 16% carbon, and thebalance 900,000 MW PEO. A film of electrode material was formed with aMayer rod onto a current collector. The separator was like the one inexample 4 but with the addition of 1% PbS.

EXAMPLE 13

A cell was tested under the conditions described in Example 6 above. Theelectrode was prepared from 50% elemental sulfur, 24% carbon, and thebalance 900,000 MW PEO and lithium trifluoromethanesulfonimide (at a49:1 weight ratio). A film of electrode material was formed with a Mayerrod onto a current collector. The separator was like the one in example4 with 900,000 MW PEO and formed with a Mayer rod.

EXAMPLE 14

A cell was tested under the conditions described in Example 4 above, butat 70° C. The electrode was prepared from 50% elemental sulfur, 24%carbon, and the balance 900,000 MW PEO and lithiumtrifluoromethanesulfonimide (at a 49:1 weight ratio). A film ofelectrode material was formed with a Mayer rod onto a current collector.The separator was like the one in example 4 with 900,000 MW PEO andformed with a Mayer rod.

EXAMPLE 15

A cell was tested under the conditions described in example 7, but withdischarge rates ranging from 0.4 to 9 mA/cm². The electrode was preparedwith 50% elemental sulfur, 16% carbon, and the balance 900,000 MW PEO. Afilm of the electrode material was formed with a Mayer rod onto acurrent collector. The separator was like the one in example 4 with900,000 MW PEO and formed with a Mayer rod. As seen from the plotpresented in FIG. 13, an extraordinarily high pulse power of 7400 W/kgof the positive electrode can be attained.

Table 1 presented in FIG. 12a summarizes the performance of therepresentative battery cells of examples 1-7 under the specific testingconditions detailed in each example. Table 2 presented in FIG. 12bsummarizes the performance of the representative battery cells ofexamples 8-14 under the specific testing conditions detailed in eachexample.

The demonstrated specific energies and specific powers listed above arebased on the entire composite positive electrode. The electrolyteseparators and lithium foils used for the laboratory tests were notoptimized for the final battery. The battery projections are based onusing 5 μm thick polymeric electrolyte separators, 30 μm thick lithiumfoil and 2.5-5.0 μm thick current collectors. Additionally, there is a10% weight increase allocated for the external casing assuming forbatteries larger than 1 Amphour.

Depending on the exact size and configuration of the cell laminate, thefinished battery performance is approximately 30-70% of the positiveelectrode film performance. For simplicity, 50% has been used for theconversion between positive electrode performance and batteryprojections (this is equivalent to 100% battery burden). The calculateddensity range of the battery ranged from 1.0-1.6 gm/cm³ depending on thespecific components and configurations. For simplicity, a density of1.25 gm/cm³ is used to calculate the projected energy density (Wh/l).

As evident from the table, the battery systems containing the positiveelectrode of this invention demonstrate exceptionally high specificenergies and exceed all now known solid-state intercalationcompound-based batteries. The cells of this invention also outperformcells which operate at much higher temperatures such as theNa/beta"--alumina/Na₂ S_(x) cell (350° C.), LiAl/LiCl, KCI/FeS₂ cell(450° C.).

It is seen that the invention provides high specific energy and powercells, the performance of which exceeds that of highly developed systemsnow known and in use. At the same time, the high energy and power areavailable at room temperature or ambient operation.

EXAMPLE 16

This example details one method of making active-sulfur electrodes ofthis invention. Initially, a three inch long piece of stainless steel(Brown Metals) was cut off of a four inch wide spool. Both sides of thesheet were then abraded with a sanding sponge to remove any insulatingcoating and ensure better electrical contact between the film and thestainless steel current collector. The abraded stainless steel currentcollector was wiped with acetone and Kim wipe EX-L until the Kim wipewas clean. A tab for electrically connecting the battery was made bycutting a section out of the stainless steel. The resulting stainlesssteel current collector was then weighed.

Next, the current collector was placed on a flat sheet of glass, and astandard 13 cm² glass casting ring was placed on the center of a 3"×3"portion of the steel current collector. Then a syringe was filled with apositive electrode slurry prepared according one of the examples above.Quickly 0.5 ml of the slurry was squirted out (or the desired volume toobtain the desired capacity per area) onto the area inside the glassring. Before the solvent evaporated, the bead of slurry was spread so asto cover the area inside the glass ring with a wet film of eventhickness. Thereafter, the film was allowed to dry for several hoursbefore removing the glass ring from the current collector. An X-actoknife was used to cut the film off of the glass ring. The currentcollector with the film was again weighed in order to obtain the weightof the positive electrode film.

Electrodes were also prepared on Teledyne stainless steel or aluminumfoils as described above but without abrading the steel or aluminumsince there are no insulating coatings as on the Brown Metals steel.

EXAMPLE 17

A stainless steel current collector was prepared as described in example15. The current collector was then placed on a smooth and flat glasssheet, and the middle of the Mayer rod ( # RDS 075 is standard now), wascentered on the edge of the current collector. Several milliliters ofslurry (as much as necessary so as to not run out of slurry) were pouredin front of the rod. With one hand holding the substrate in place on theglass and the other holding the middle of the rod, the rod was draggedacross the current collector leaving a wet film. The film was then driedand the process was repeated from the other end. The solvent content ofthe slurry was adjusted so that the wet film did not run (too muchsolvent) and did not have a ridged or raked appearance. When the filmwas dried, it was placed on a glass ring (at the center of the 3"×3"current collector), and a circular section was cut along the insidecircumference of the ring. The excess film outside the circle was thenscraped off and the weight of the film was determined.

EXAMPLE 18

Initially, an aluminum foil current collector prepared as in example 15was placed on a sheet of glass, and taped to the ends of the glass sothat it did not move while moving the Mayer rod. A Mayer Rod was placedon one end of the current collector and enough slurry to cover thedesired area of current collector was squirted from a syringe in frontof the Mayer Rod and onto the current collector. When the film was dry,the process was repeated as before but processed with a Mayer rod from adifferent end. When the film was dry, unwanted film was scraped off, andthe current collector was trimmed to the desired area.

EXAMPLE 19

The following procedure was employed to prepare a positive electrodeslurry having 50 wt % elemental sulfur, 16 wt % acetylene black, 2 wt %Brij 35, and 32 wt % 900,000 MW PEO. A 38×38 mm stir cross was placed inan 8 oz. Quorpac bottle (BWR Scientific, Brisbane, Calif.) with a Teflonlined top. To the bottle the following were added: 230 ml ofacetonitrile. (Aldrich HPLC grade), 6 g of sublimed and ball milledsulfur powder (Aldrich), 1.93 g of acetylene (Shawinigin) carbon black(Chevron Cedar Bayou Plant), and 0.24 g of Brij 35 (Fluka). The contentsof the bottle were then stirred overnight on a magnetic stir plate. Thestir-plate power was set to stir at as high an RPM as possible withoutsplattering or sucking air. The next day, as the slurry was rapidlystirring, 3.85 g of 900,000 MW PEO (Aldrich) was added in a stream so asnot to form a few large lumps of solvent swollen PEO but rather manytiny lumps of solvent swollen PEO. During the next two days, the speedof the stir bar was adjusted to maintain as high as possible rpms, againwithout splattering or sucking air. After stirring for two nights, thePEO was dissolved and the slurry was used to prepare thin films byeither ring casting or Mayer rod techniques.

Alternatively, sublimed and precipitated sulfurs were used instead ofthe ball milled sulfur described above, but instead of mixing for twonights, about two weeks of stirring were required. If the slurry ismixed for only two nights the resulting thin film was found to be porousand lumpy.

EXAMPLE 20

The following procedure was employed to prepare a positive electrodeslurry having 50 wt % elemental sulfur, 24 wt % acetylene black, 2 wt %Brij 35, and the balance 900,000 MW PEO:lithiumtrifluoromethanesulfonimide (20:1) in acetonitrile (ml AN:gm PEO, 90:1).A 38×38 mm stir cross was placed in an 8 oz. Quorpac bottle (BWRScientific, Brisbane, Calif.) with a Teflon lined top. To the bottle thefollowing were added: 0.59 g lithium trifluoromethanesulfonimide (addedin a dry box), 200 ml of acetonitrile (Aldrich HPLC grade), 5 g ofsublimed and ball milled sulfur powder (Aldrich), 2.4 g of acetylene(Shawinigin) carbon black (Chevron Cedar Bayou Plant), and 0.2 g of Brij35 (Fluka). The contents of the bottle were then stirred overnight on amagnetic stir plate. The stir-plate power was set to stir at as high anRPM as possible without splattering or sucking air. The next day, as theslurry was rapidly stirring, 1.8 g of 900,000 MW PEO (Aldrich) was addedin a stream so as not to form a few large lumps of solvent swollen PEObut rather many tiny lumps of solvent swollen PEO. During the next twodays, the speed of the stir bar was adjusted to maintain as high aspossible rpms, again without splattering or sucking air. After stirringfor two nights, the PEO was dissolved and the slurry was used to preparethin films by either ring casting or Mayer rod techniques.

EXAMPLE 21

The following procedure was employed for various slurry compositions(identified below). Initially, a glass bottle and a stir bar were washedwith acetone, and the stir bar was placed in the jar. Then anappropriate amount of acetonitrile (depending upon subsequentprocessing) was added to the jar and the bottle was capped. The bottlewith its contents was placed onto a stir plate operated at sufficientpower to create a vortex in the acetonitrile.

Next, PEO was measured and slowly added to the bottle while it was stillon the stir plate. The PEO was introduced in very small amounts tomaximize the contact with acetonitrile and promote rapid mixing. If saltwas added, it was measured and added in the same fashion as the PEO. Ifthere were other solubles (brij) to be added, they were also mixed inalso at this point. All components were mixed until dissolved. Next, allinsoluble materials including sulfur and carbon were measured and addedto the mixture. Mixing was conducted for a minimum of two days.

The slurry combinations employed were as follows:

(A) 50 wt % elemental sulfur; 24% Carbon (Acetylene Black); 2% brij 35;24% (20 moles PEO 900K to 1 mole lithium trifluoromethanesulfonimide)with 90 ml acetonitrile per gram of PEO.

(B) 50 wt % elemental sulfur; 16% Carbon (Acetylene Black); 1% brij 35;33% PEO 900K with 60 ml acrylonitrile per gram of PEO.

Ranges of components used in preparing various compositions inaccordance with this example are as follows: 24%-55% wt elementalsulfur; 8%-24% wt Carbon (Acetylene Black); 30 ml acrylonitrile per gramPEO to 110 ml acetonitrile per gram PEO; and 40, 60 and 90 ml water pergram PEO. Other compositions had various additives pegged to elementalsulfur according to the following: (1) 5 wt % Brilliant Yellow Dyeadditive with 55% elemental sulfur; (2) 5%2,5-dimercapto-1,3,4-dithiadiazole with 45% elemental sulfur; (3) 2 wt %lithium iodide with 48% elemental sulfur; (4) 5 wt % iodine with 50%sulfur; (5) 1 wt % PbS with 49% elemental sulfur; and (6) 5 wt %polyethylene dithiol with 45% elemental sulfur.

The foregoing describes the instant invention and its presentlypreferred embodiments. Numerous modifications and variations in thepractice of this invention are expected to occur to those skilled in theart. Such modifications and variations are encompassed within thefollowing claims.

All references cited herein are incorporated by reference.

What is claimed is:
 1. A battery cell comprising:a) a negative electrodeincluding a metal or an ion of the metal; b) a positive electrodecomprisingi) an electrochemically active material comprising sulfur inthe form of at least one of elemental sulfur, a sulfide of the metal,and a polysulfide of the metal, and ii) an electronically conductivematerial; and c) a gel-state or polymeric solid-state electrolyteseparator electronically separating the positive and negativeelectrodes, d) wherein at least about 10% of the sulfur is accessible toelectrons and ionic charge carriers.
 2. The battery cell of claim 1,wherein the battery cell is rechargeable.
 3. The battery cell of claim2, wherein at least about 10% of this sulfur is accessible to electronsand ionic charge carriers over at least about 50 successive cycles. 4.The battery cell of claim 3, wherein at least about 50% of the sulfur isaccessible to electrons and ionic charge carriers that over at leastabout 2 successive cycles.
 5. The battery cell of claim 1, wherein themetal of the negative electrode includes at least one of sodium andlithium.
 6. The battery cell of claim 5, wherein the metal in thenegative electrode is a lithium metal electrode.
 7. The battery cell ofclaim 1, wherein the negative electrode is a lithium intercalationelectrode.
 8. The battery cell of claim 1, wherein the electronicconductor of the positive electrode is at least one of carbon and anelectronically conductive polymer.
 9. The battery cell of claim 8,wherein the positive electrode further comprises an polymeric disulfidematerial in which the disulfide bond is located in the material'sbackbone.
 10. The battery cell of claim 1, wherein the electrolyteseparator comprises a gel-state electrolyte separator including at least20% by weight of an aprotic organic liquid immobilized by the presenceof a gelling agent.
 11. The battery cell of claim 1, wherein theelectrolyte separator comprises a polymeric solid-state electrolyteseparator.
 12. The battery cell of claim 11, wherein in the polymericsolid-state electrolyte separator comprises a polymer of ethylene oxide.13. The battery cell of claim 1, wherein the positive electrode furthercomprises an ionic conductor.
 14. The battery cell of claim 1, whereinthe electrolyte separator comprises an aprotic organic liquid.
 15. Abattery cell comprising:a) a negative electrode including a metal or anion of the metal; b) a positive electrode comprisingi) anelectrochemically active material comprising sulfur in the form of atleast one of elemental sulfur, a sulfide of the metal, and a polysulfideof the metal, ii) an electronically conductive material, and iii) aliquid state ionic conductor, and c) a gel-state or polymericsolid-state electrolyte separator electronically separating the positiveand negative electrodes, d) wherein at least about 10% of the sulfur isaccessible to electrons and ionic charge carriers.
 16. The battery cellof claim 15, wherein the battery cell is rechargeable.
 17. The batterycell of claim 16, wherein at least about 10% of this sulfur isaccessible to electrons and ionic charge carriers over at least about 50successive cycles.
 18. The battery cell of claim 17, wherein at leastabout 20% of the sulfur is accessible to electrons and ionic chargecarriers that over at least about 50 successive cycles.
 19. The batterycell of claim 18, wherein at least about 50% of the sulfur is accessibleto electrons and ionic charge carriers that over at least about 2successive cycles.
 20. The battery cell of claim 15, wherein the metalof the negative electrode includes at least one of sodium and lithium.21. The battery cell of claim 20, wherein the metal in the negativeelectrode is a lithium metal electrode.
 22. The battery cell of claim15, wherein the electronic conductor of the positive electrode is atleast one of carbon and an electronically conductive polymer.
 23. Thebattery cell of claim 22, wherein the positive electrode furthercomprises an organodisulfide material in which the disulfide bond islocated in the materials backbone.
 24. The battery cell of claim 15,wherein the electrolyte separator comprises a gel-state electrolyteseparator including at least 20% by weight of an aprotic organic liquidimmobilized by the presence of a gelling agent.
 25. The battery cell ofclaim 15, wherein the electrolyte separator comprises a polymericsolid-state electrolyte separator.
 26. The battery cell of claim 25,wherein in the polymeric solid-state electrolyte separator comprises apolymer of ethylene oxide.
 27. The battery cell of claim 15, wherein theliquid state ionic conductor includes at least one of sulfolane,dimethyl sulfone, a dialkyl carbonate, tetrahydrofuran, dioxolane,propylene carbonate, ethylene carbonate, dimethyl carbonate,butyrolactone, N-methylpyrrolidinone, tetramethylurea, glymes, ethers, acrown ether, and dimethoxyethane.
 28. The battery cell of claim 15,wherein the electrolyte separator includes a gelling agent and at leastone of sulfolane, dimethyl sulfone, a dialkyl carbonate,tetrahydrofuran, dioxolane, propylene carbonate, ethylene carbonate,dimethyl carbonate, butyrolactone, N-methylpyrrolidinone,tetramethylurea, glymes, ethers, a crown ether, and dimethoxyethane.